Adjustable inductance



July 2l, 1942.

N. P. CASE ADJUSTABLE INDUCTA NGE Original Filed March 3l, 193'? INVENTOR.

J NELSON P. @SE BY MMM/ ATTORNEY,

Reissued July 21, 1942 ADJUSTABLE INDUCTANCE Nelson P. Case, Great Neck. N. Y., assigner to Hueltine Corporation, a corporation or Delaware l Original No. 2,173,084, dated September 10, 1939,

Serial No. 134,069, March 31, 1937.

Mileation for reissue June 27, 1941, Serial No. 400,153

20 Claims.

This invention relates to adjustable inductance units which, while of general application, are especially adapted for use as tuning elements in the resonant circuits of a high-frequency signaling system for tuning the circuits over broad bands of frequencies.

In high-frequency signaling systems, it is generally desirable to maintain the antiresonant impedances of the resonant circuits at or near their maximum values as they are tuned over their respective ranges of frequencies in order that high gain may be obtained under all conditions. When an adjustable condenser is utilized as a tuning element of a tunable circuit. its capacitance becomes so large at the low-frequency end of the tuning range that the L/C ratio and the antiresonant impedance of the circuit, and hence the gain, are appreciably reduced. However, if fixed condensers and adjustable inductance tuning elements are utilized in such resonant circuits. a high antiresonant impedance may be obtained over the entire tuning range.

Adjustable inductance units heretofore devised have been subject to various objections. In general, these units have been either of the type comprising fixed windings and an associated adjustable shield or of the well-known adjustable winding variometer type. Both of these types ordinarily are objectionable in that they have relatively small ranges ,of inductance variation and the shield type also has relatively high power factors. While units of the variometer type have usually been the least objectionable in these respects, even they ordinarily have a relatively limited range of variation. For an adjustable reactance element suitable for tuning over a wide frequency range, such as the broadcast band, a variation of approximately 9:1 is required. Further, in conventional inductance units of the variometer type. very close spacing between windings is required for units of moderate inductance and moderate dimensions, giving a large variation of distributed capacitance and limiting the tuning range when such unit is used as a tuning element of a resonant circuit. Moreover. inductance units of the variometer type heretofore devised have been relatively complicated and otherwise mechanically objectionable.

It is an object of the present invention, therefore, to provide a novel and improved adjustable inductance unit having a wide range of inductance variation.

It is a further object of the invention to provide an adjustable inductance unit of the type described having one or more of the following characteristics: simplicity. compactness, ruggedness, and ease of operation.

In accordance with the present invention, there is provided a composite adjustable inductance unit which comprises a ilrst substantially cylindrical winding means providing a iirst magnetic iield of predetermined shape. intensity. and polarity. A second substantially cylindrical winding means, interleaving with said ilrst winding means and having a mean diameter equal to that of said ilrst winding means, is arranged to provide a second magnetic neld coaxial with. of substantially the same shape and intensity as, but ot an opposite polarity to, that of the first-mentioned winding means. An adjusting means is provided for moving the two winding means with respect to each other. This relative movement is preferably from a first position. in which the two magnetic fields are substantially mutually exclusive and a maximum inductance is provided by the unit, to a 'second position, in which the fields are substantially coincident. so that the field of the second winding means substantially neutralizes or cancels the eld oi the first-mentioned winding means and a minimum inductance of a relatively small value is provided by .the unit.

In one approved embodiment of the invention, three windings are individually disposed on three coaxial cylindrical forms. These forms are dimensioned for telescopic movement, one within the other two. The windings are so proportioned and arranged that the two windings on the outer and inner forms provide a first magnetic field as described above. and the windlng on the intermediate form provides the second magnetic field. Preferably. the outer and inner forms are secured in relatively fixed telescopic relation, and means are provided operatively interconnecting a control element and the intermediate form so that equal adjustments of the control element over its range effect different displacements of the intermediate form relative to the outer and inner forms to vary the total inductance of the unit as described above. I'his manner of control renders the unit especially adaptable for use as a tuning element, in that equal adjustments of the control element may be made to eii'ect equal variations in the resonant frequency of a circuit in which the unit is 'Jsed as a tuning element.

As used in this specification. the term mean diameter" is used to denote the edective diameter ior purposes of inductance computation o! winding systems to which the term is applied, and thus it may apply to a single layer winding. a multilayer winding. or a multiwinding system.

Also as used in this specification, the term "cylindrical winding" is intended to include windings, sometimes referred to as solenoids, and having any desired ratio of length to depth of winding.

For a better understanding oi' the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a simplined circuit diagram illustrating one approved form of the present invention; Fig. 2 is a longitudinal sectional view of the windings of an inductanoe unit such as is illustrated in Fig. l, indicating the magnetic fields produced by the windings; Figs. 3, 4, 5, and 6 are circuit diagrams illustrating various modiiled forms oi the present invention; and Fig. 7 is a side elevation of a composite coupling unit embodying the present invention with certain portions shown in longitudinal section and certain portions broken away.

Referring in detail to Figs. 1 and 2, the inductance unit of the present invention illustrated by these figures comprises a pair of coaxial helical windings IU and il, disposed one within the other with an annular space therebetween, and an additional helical winding I2 coaxial with, and adapted to be telescoped within the annular space between, the windings III and II, the windings III and II and the winding I2 thus being interleaving. The cross sections of the windings may, :of course, be of any desired shape other than circular, as shown. The radial dimension of the annular space between the windings III and II is preferably a small fraction of the mean diameter of the windings so that the magnetic ileld produced jointly by the two windings I and II is of substantially the same shape and intensity' as that produced by the single winding I2. For this purpose also, it is preferable that the diameter oi the wire used for the windings I0 and I I be small relative to the mean diameter of the windings. The winding I2 is thus movable relative to the rst winding means, comprising windings III and Il, so that at least portions thereof have a substantial axial component of movement with reference to the axis oi the first winding means.

In the arrangement of Fig. l, the winding I2 is wound in a direction opposite to that of the windings III and II so that the polarity of the ield of the winding I2 is opposite to that of the field o! the other two windings. The windings III and II are connected in parallel and the winding I2 is connected in series therewith. The three windings are so proportioned as to have substantially equal inductances so that the current through the winding I2 divides equally between the windings III and II. By virtue of this arrangement, as illustrated in Fig. 2, the windings III and II jointly provide a iirst magnetic iield of a predetermined shape, intensity, and polarity. while the winding I2 provides a second magnetic fieldI of substantially the same shape and intensity as.' but of an opposite polarity to, that of the ilrst ileld. As indicated by the arrow in Fig. 1, the

winding I2 is rectilinearly or axially movable relativo to the windings Il and II from a'iirst position external to these windings, in which the two nelds are substantially mutually exclusive, to a second position telescopically interposed between the windings Il and I I. in which the two nelds are substantially coincident.

The inductance of the unit described above may be determined according to the formula:

L I M =+2l+h2im (l) where L is the total inductance of the unit; L1, In, and La are the self-inductances of the windings Ill, II, and I2, respectively; M1 is the mutual inductance between the windings III and II; and Mz is the mutual inductance between the pair oi windings III and II and the winding i2. The dimensions of the unit are so related that the maximum value of 2M; approaches that of the expression- Le. L1 +112 'as closely as possible.

It will thus be seen that, with the winding I2 disposed in the position in which the two ilelds are substantially mutually exclusive, there is substantially no mutual inductance between the winding I2 and the windings I0 and II, the value of Mz is inappreciable or substantially zero. In this position, therefore, the total inductance of the unit is a maximum. On the other hand, when the winding I2 is disposed in the secondmentioned position with the two fields substantially coincident, the mutual inductance M2 between the winding I2 and the windings Ill and Il is a maximum and the total inductance of the unit is reduced to a minimum.

It will be noted that, in the arrangement of Fig. 1, the high potential end of the winding I2 is adjacent the high potential ends of the windings I0 and II when the winding I2 is disposed within the other two windings. By virtue of this arrangement, the differences in potential of adjacent portions of the winding I2 and the other two windings is fairly small for all adjustments `thereof and the variation of distributed capacirance between these windings has only a small ellect on the characteristics of the unit. Care Y should be taken that the power factors and impedances of the windings III and II are substantially equal at all frequencies at which the unit is to be operated so that the current through the two windings will be substantially equal under all conditions.

In Fig. 3 there is illustrated a modied form of the invention which is identical with that shown in Fig. l, except that all three of the windings are wound in the same direction and the dierence in polarities of the two fields is obtained by reversing the connections of the winding I2. While, in certain cases, substantially satisfactory results may be obtained with this arrangement, when the windings are completely telescoped, the low potential end oi winding I2 is opposite to the high potential ends of the other two windings, so that the distributed capacitance therebetween is somewhat more effective than in the embodiment of Fig. 1.

In Figs. 4 and 5 two additional modified forms of the invention are illustrated which are similar to the embodiments of Figs. l and 3, respectively, except that all three of the windings are connected in series and the several windings are of correspondingly different relative proportions. In Fig. 4 the difference in polarity of the two fields is obtained, as in Fig. l, by winding the element oppositely to the windings III and II. In Fig. 5, on the other hand, the difference in polarity is obtained, as in Fig. 3, by reversing the connections of the winding I 2. In the ernbodiments of both Figs. 4 and 5, since all of the windings are connected in series, the total inductance provided by the unit may be according to the formula:

in which corresponding terms represent the same factors as in Equation (l). of Figs. 4 and 5, also, the inductances of the windings I and II individually are preferably substantially equal, but. since the total inductance of these winding, L1+L2+2Mi, is approximately 4Li, the total inductance of the unit is Therefore, it is necessary that the inductance of the winding I2 be approximately four times that of each of the windings I0 and II in order that the total inductance may be reduced to approximately zero when the two fields are substantially coincident or Mz is at its greatest value.

In Fig. 6 still another modified form of the invention is illustrated. Here, the three windings are all connected in parallel with the winding I2 wound in a direction opposite to that of the windings III and II in order to obtain the desired difference in polarity of the two fields. In this case, the winding I2 preferably has a self-inductance approximately equal to that of each of the windings IIJ and II in order that the total inductance of the unit may be varied from its maximum value to a minimum value of approximately zero.

The inductance of the unit of Fig. 6 may be determined according to the formula:

in which corresponding terms respresent the same factors as in the other equations.

If then, L1=Lz=La=M1=M2=Li where unity coupling between Li and L: is assumed, then for the minimum total inductance of the unit the following equation holds:

and for the maximum total inductance for the unit, where M2=0, the following equation holds:

LN2 L! L max. =2 -2- (6) It will be noted that this embodiment has the special advantage that the effect of the distributed capacitance is reduced to a vminimum, since, when the windings are completely telescoped, there is no difference in potentials between adjacent parts of the windings.

Referring to Fig. 7, there is illustrated a physical construction of an inductance unit embodying the present invention. This unit comprises three coaxial cylindrical forms I3, Il, and I5 on which are disposed the windings I0, II, and I2, respectively. The form I4 is disposed within the In the embodiments form I5, leaving an annular space therebetween and these two forms are rigidly secured to a suitable base I6, as shown. The form I5 is of a diameter intermediate between the diameters of the forms I3 and Il and is adapted to be telescoped therebetween. An end piece II is secured to the end of the form I5 remote from the base I8 and is provided with guide passages III and I9. There is also provided a fixed support 20 having transverse openings 2l and 22 corresponding to the openings I8 and I5, respectively, in the end piece I'I. A guide rod 23 is rigidly secured in the opening 2| parallel to the common axis of the forms and extends through the passage I8 to prevent rotation of the form I5 while guiding axial movements thereof. A shaft 24 extends through the opening 22 and passage I9 and is provided with a suitable actuating knob 25 and annular collars 26 to prevent longitudinal movement thereof. A spiral screw thread 2l, of a pitch gradualhf increasing in the direction of the support 2li, is formed in the shaft 24 and engages a pin 28 in the end piece I1. This shaft and pin arrangement thus provides a nonuniform motion-transmitting means connecting the control element or knob 25 and the intermediate form. The windings III, II, and I2 may be proportioned, arranged, and connected in accordance with the embodiments illustrated in any ofFigs. l, 3, 4, 5 or 6.

In the operation of the inductance unit of Fig.

` 7 to adjust the inductance thereof, the knob 25 is rotated to rotate the shaft 24 and, because of the engagement of the pin 2B in the screw thread 21, the form I5 is axially moved relative to the forms I3 and Il. The limits of movement of the form I5 are from its innermost position where the fields developed by the windings are substantially coincident and the total inductance of the unit is a minimum, t0 a position external to the forms I3 and Il where the magnetic fields in question are substantially mutually exclusive and the total inductance of the unit is a maximum. The forms are so proportioned that the radial dimension of the annular space between the forms I3 and I4 is a small fraction of the mean diameters of these forms. By virtue of this arrangement, the magnetic fields produced jointly by the windings on these two forms is of substantially the same shape and intensity as that produced by the winding on the form I5 and when the form I5 is telescopically moved to its position between the other forms, the two fields are substantially coincident. It will be apparent that this result is not obtainable by a unit including only two telescopically engaging windings since, inasmuch as one of such windings must have a lesser diameter than the other in order to obtain telescopic engagement. the two fields will necessarily be of different shapes rendering coincident positioning thereof impossible. Due to the variation in the pitch of the screw thread 2l, equal adjustments of the knob 25 from the first to the second extreme positions mentioned above effect gradually increasing adjustments of the winding I2. A unit of the type described is especially well adapted for use as a tuning element in a resonant circuit since, with the pitch of the screw thread varying in a proper manner, equal adjustments of the knob 25 effect the proper adjustments of the total inductance of the unit tc impart to the unit a straight-line frequency characteristic over its entire tuning range.

While there have been described what are at present considered the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from this invention, including various other manners of connecting and positioning the windings of the unit in addition to those described in detail, and, therefore. it is aimed in the appended claims to cover all such changes and modificationsV as fall within the true spirit and scope ci this invention.

What is claimed is:

l. A composite adjustable inductance unit comprising a iirst substantially cylindrical winding means providing a magnetic ileld of predetermined shape, intensity, and polarity, a second substantially cylindrical winding means interleaving with said iirst winding means and having a mean diameter equal to that of said first winding means and providing a magnetic field coaxial with, of substantially the same shape and intensity as, but of an opposite polarity to, that of said first means, and means for relatively axially moving said two winding means.

2. A composite adjustable inductance unit comprising a first substantially cylindrical winding means providing a magnetic field of predetermined shape, intensity, and polarity, a second substantially cylindrical winding means interleaving with said ilrst windingr means and having a mean diameter equal to that of said first winding means and providing a magnetic ileld coaxial with, of substantially the same shape and intensity as, but of an opposite polarity to, that of said first means, and means for relatively axially moving said two winding means from a rst position in which said fields are substantially mutually exclusive to provide a maximum inductance to a second position in which said ilelds substantially coincide to provide a minimum inductance.

3. A composite adjustable inductance unit comprising a first substantially cylindrical winding means providing a magnetic fleld oi' predetermined shape, intensity, and polarity, a second substantially cylindrical winding means interleaving with said first winding means and having a mean diameter equal to that of said ilrst winding means and providing a magnetic eld coaxial with. of substantially the same shape and intensity as, but of an opposite polarity to, that of said first means, adjusting means for relatively axially moving said two winding means, control means i or said adjusting means, and nonuniiorm motion-transmitting means connecting said control means and said adjusting means whereby equal adjustments of the former over its range of adjustment effect predetermined different relative movements oi said winding means.

4. A composite adjustable inductance unit comprising a first substantially cylindrical winding means providing a magnetic ileld of predetermined shape, intenslty, and polarity. a second substantially cylindrical winding means interleaving with said rst winding means and having a mean diameter equal to that of said i'lrst winding means and providing a magnetic iield of substantially the same shape and intensity as. but of an opposite polarity to, that of said first means, adjusting means for relatively moving said two winding means with a motion having a substantial axial component while maintaining the polarities of said nelds substantially unchanged from a ilrst position with the iields of exclusive to a second position with the fields o! said winding means substantially coincident, control means for said adjusting means, and nonuniform motion-transmitting means so connecting said control means and adjusting means that equal adjustments o! said control means effect gradually decreasing relative movements as said windings are moved from said iirst position to said second position.

5. A composite adjustable inductance unit comprising a rst substantially cylindrical winding means providing a magnetic iield of predetermined shape, intensity, and polaritya second substantially cylindrical winding means interleaving with said ilrst winding means and having a mean diameter equal to that of said rst winding means and providing a magnetic ileld oi' the same shape and intensity as, but of an opposite polarity to, that of said rst means, said two winding means being coaxially disposed and dimensioned for telescopic movement, and means for relatively axially moving said winding means.

6. A composite adjustable inductance unit comprising a pair of relatively fixed substantially laid two winding means substantially mutually 76 cylindrical windings providing a magnetic ileld of a predetermined shape, intensity, and polarity, an additional substantially cylindrical winding interleaving with said pair of windings and having a diameter equal to the mean diameter o! said pair of windings and providing a magnetic field of substantially the same shape and intensity as, but oi' an opposite polarity to, that of said pair oi windings, and means for axially moving said pair of windings and said additional winding relative to each other.

7. A composite adjustable inductance unit comprising a pair of relatively iixed coaxial substantially cylindrical windings one disposed within the other, an additional substantially cylindrical winding interleaving with said pair of windings and having a diameter equal to the mean diameter ol' said pair of windings and coaxially disposed with respect to said pair of windings and dimensioned for telescopic movement therebetween, and means for telescopically moving said pair or windings and said additional winding relative to each other from a substantially mutually exclusive position to provide maximum inductance to a position with said additional winding telescoped between the windings of said pair to provide minimum lnductan ce.

8. A composite adjustable inductance unit comprising s. pair of coaxial substantially cylindrical windings disposed one within the other and providing an annular space therebetween of a radial dimension which is a small fraction of the mean diameter of said windings. an additional winding coaxial with, and interleaving with, said pair of windings and dimensioned for telescopic movement therebetween, and means for relatively telescopically moving said pair o! windings and said additional winding from a substantially mutually exclusive position provid ing maximum inductance to a position with said additional winding disposed in said annular space to provide minimum inductance. y

9. A composite adjustable inductance unit comprising a ilrst substantially cylindrical winding means, a second interleaving substantially cylindrical winding means, said two winding means having the same mean diameter and being coaxially disposed and dimensioned for telescopic engagement, and means for axially moving said two winding means, relative to each other, from a first position with said two windlng means substantially mutually exclusive to provide a maximum inductance, to a second position with said two winding means telescopically disposed to provide a minimum inductance, the high potential ends of both winding means being substantially adjacent in the last-mentioned positlon.

10. A composite adjustable inductance unit comprising a first substantially cylindrical winding means providing a magnetic geld of a predetermined shape, lntensity. an polarity, a second substantially cylindrical winding means interleaving with said first winding means and having a mean diameter equal to that of said first winding means and connected in series with said first means, said two winding means providing a magnetic field coaxial with, of substantially the same shape and intensity as, but of an opposite polarity to, that of said first means, and means for relatively axially moving said two winding means.

1l. A composite adjustable inductance unit comprising a first substantially cylindrical winding means providing a magnetic field of predetermined shape, intensity, and polarity, a second substantially cylindrical winding means interleaving with said first winding means and having a mean diameter equal to that of said first winding means and connected in parallel with said first means, said two winding means of substantially the same shape and intensity as, but of apolarity opposite to, that of said pair of windings, and means for relatively axially moving said pair of Vwindings and said additional winding.

15. A composite adjustable inductance unit comprising a pair of concentric substantially cylindrical windings connected in series, having substantially equal inductances and providing a magnetic field of predetermined polarity, an additional substantially cylindrical winding interleaving with said pair of windings and having a diameter equal to the mean diameter of said pair oi windings and connected in series and concentric with the windings of said pair, having an inductance approximately four times that of each of the windings of said pair and providing a second magnetic field coaxial with, and of a polarity opposite to, that ,of the first said field, and means for relatively axially moving said pair of windings and said additional winding from positions with said field substantially mutually exclusive to positions with said fields substantially coincident.

16. A composite adjustable inductance unit comprising three coaxial cylindrical forms adapted to be telescoped one within another, the

, inner and outer forms being relatively fixed and providing a magnetic field coaxial with, of subr stantially the same shape and intensity as, but of an opposite polarity to, that of said first means, and means for relatively axially moving said two Winding means.

l2. A composite adjustable inductance unit, comprising a pair of substantially cylindrical windings connected in parallel and arranged to provide a magnetic field of a predetermined shape, intensity, and polarity, an additional lnterleaving substantially cylindrical winding connected in series with the windings of said pair and arranged to provide a magnetic field coaxial with, of substantially the same shape and intensity as, but of a polarity opposite to, that of said pair of windings, and means for relatively axially moving said pair of windings and said additional winding.

13. A composite adjustable inductance unit comprising a pair of concentric substantially cylindrical windings connected in parallel and providing a magnetic field of predetermined polarity, an additional interleaving substantially cylindrical winding connected in series and concentric with the windingsof said pair and providing a magnetic field coaxial with, but of a polarity opposite to, that of the first said field, all of said windings being proportioned to have substantially the same inductance, and means for relatively axially moving said pair of windings and said additional winding from positions with said fields substantially mutually exclusive to positions with said elds substantially coincident.

14. A composite adjustable inductance unit comprising a pair of substantially cylindrical windings connected in series and arranged to provide a magnetic field of a predetermined shape, intensity, and polarity, an additional substantially cylindrical winding interleaving with said pair of windings and having a diameter equal to the mean diameter of said pair of windings and connected in series with the said pair of windings, said additional winding being arranged to provide a magnetic field coaxial with,

having a mean diameter equal to that of the other of said forms, a winding disposed on each of said forms, said windings being connected in circuit and so dimensioned and disposed that the windings on the outer and inner forms provide a magnetic field of predetermined shape, intensity, and polarity and the winding on the intermediate form interleaves with the windings on the inner and outer forms and provides a magnetic field of substantially the same shape and intensity as. but of an opposite polarity to, that of said first field. and means for effecting relative movement between said outer and inner forms and said intermediate form.

1'?. A composite adjustable inductance unit comprising three coaxial cylindrical forms adapted to be telescoped one within another, the inner and outer forms being relatively fixed and having a mean diameter equal to that of the other of said forms, a winding disposed on each of said forms, said windings being connected in circuit and so dimensioned and disposed that the windings on the outer and inner forms provide a magnetic field of predetermined shape, intensity, and polarity and the winding on the intermediate form interleaves with the windings on the inner and outer forms and provides a magnetic field of substantially the same shape and intensity as, but of an opposite polarity to, that of said first field, a control element, and nonuniform motiontransmltting means connecting said element to said intermediate form, whereby equal adjustments of said element over its range of adjustment effect different movements of said intermediate form relative to said outer and inner forms.

18. A composite adjustable inductance unit comprising three coaxial cylindrical forms adapted to be telescoped one within another, the inner and outer forms being relatively fixed and havring a mean diameter equal to that of the other of said forms, a winding disposed on each of said forms, said windings being connected in circuit and so dimensioned and disposed that the windings on the outer and inner forms provide a magetic field of predetermined shape,

intensity, and polarity and the winding on the intermediate form interleaves with the windings ontheinnerand outerformsandprovidesa magnetic neld oi' substantially the same shape and intensity as, but o! an opposite polarity to. that of said iirst eld, a control element, and nonuniiorm motion-transmitting means so connecting said element and said intermediate form that equal adiusiments of said element over its range ot adjustment ei'iect gradually decreasing movements oi said intermediate iorxn relative to said outer and inner forms from s nrst position substantially external' to said outer and inner forms to a second position substantially completely telescoped therebetween. l

191A composite adjustable inductance unit comprising a ilrst substantially cylindrical winding means providing a magnetic deld of predetermined shape. intensity and polarity. a second substantially cylindrical winding means adapted to interleave with said iirst winding means and movable relative to said iirst winding means so that at least portions thereof have a substantial axial component oi' movement with reference to the axis oi' said nrst Vwinding means. said second winding means having a mean diameter equal to that oi' said-tiret winding means and provid- IBJBO ing in one adjustment oi' the unit a magnetic neid coaxial with. of substantially the same shape and intensity as, but o! an opposite polarity to. that o! said nrst means, and means for eiiecting relative movement between said two winding means from the positions corresponding to said one adjustment.

20. A composite adjustable Ainductance unit comprising, a pair of relatively nxed substantially l cylindrical windings providing a maanetic ileld of a predetermined shape,intensity and polarity, an additional substantially cylindrical winding interleaving with said pair of windings and movable relative to said pair oi' windings so that at least portions thereof have a substantial axial component oi' movement Fwith reference to the axis oi said pair oi' winding means. said additional winding having a diameter equal to the mean diameter oi' said pair oi windings and providing a magnetic ileld oi substantially the same shape and intensity as, but of opposite polarity to, that of said pair of windings, and means for enecting relative movement between said pair of windings and said additional winding.

NELSON P. CASE.

. CERTIFICATE on CORRECTION. s Reissue No. 22,159. July 21', 19'42.

mson P. CASE.

I-t is Yhereby certified that error. appears `in the printed specification of the above numbered patent requiring Correction as follows: Pageh., seo

ond Column, line 58, Claim 8, before "winding'l insert --mbstnntially vcyl ndricBl--ypage 5,second column, line 71|.,'c1a-im 18, for "magst-io read y --magnetic--g and that the. seid Letters Patent: should be read with this correction' therein that the same may coform tothe record ofthe case in the patent office.

Signed and sealed this 15th day of September, A. D. 1%2.

Henry Van Arsdale, (Seal) Acting Commissionervof Patents. 

